Interest in the polymers of Group 14 elements has historically been concerned primarily with those containing carbon. The carbon-based polymers have demonstrated a remarkable chemical versatility and have found a splendid variety of applications. Although promising in their own respects, polymers based on the heavier Group 14 elements (silicon, germanium, tin, and lead) have been less studied.
The commercial potential for polymers based on the heavier Group 14 elements is illustrated by that for some recently developed polysilane high polymers (Miller et al.). For instance, soluble, castable polysilane homo- and copolymers have been developed. Also, the low-temperature pyrolysis (Yajima process) of polydimethylsilane leads to the formation of a soluble carbosilane which can be processed into forms, films, or fibers. The soluble carbosilanes can be further processed and fired to produce .beta.-SiC fibers, which are otherwise not available.
An important respect in which heavy Group 14 polymers differ from carbon-based polymers is in their photolability and sensitivity to ionizing radiation. These properties are reflected in facile chain scission and crosslinking upon exposure to sunlight. For instance, heavy group 14 polymers have been employed as broad spectrum photoinitiators of vinyl polymerizations. They often have excellent photoconductive properties and by adjusting doping levels both conductive and semiconductive films have been made.
Perhaps the most extensively explored application of heavy Group 14 polymers, e.g., polysilanes, is as photoresists in microlithography. The polysilane polymers are soluble in organic solvents and can be coated as high quality optical films onto a semiconductor substrate. They are thermally and oxidatively stable but photolabile over a broad spectral range and their resistance to oxygen plasma conditions makes them well-suited to multilayer lithographic applications, where highly anisotropic etch profiles are desired. They also are attractive as contrast enhancing agents in microlithography, photoresists in e-beam lithography, ablative targets in dry development schemes, and as materials for nonlinear optical purposes.
Thin films containing tin can also be used as conductors or coatings for semiconductors, typically in the form of elemental metal or metal oxides. Such tin-containing films can be formed by ion or electron beam induced deposition using tin tetrachloride or tetramethyltin (Funsten et al.). Films produced by these methods generally retain an undesirable amount of carbon, hydrogen, and/or halogen from the precursor molecules as high conductivities are found to require significant loss of these impurities. To the extent compounds having a high tin content can be exploited in various applications involving their decomposition, such compounds may parallel the polysilanes or even be superior to previously used materials.
Among the approaches taken to forming compounds having a high tin content are those that generate a tin-tin bond in the compound. [See, e.g., Organotin Chemistry, Elsevier, I. Omae, ed., New York, N.Y. (1989)]. Such approaches include those involving pyrolysis, as well as Grignard and Wurtz coupling schemes. For instance, Reifenberg et al. (U.S. Pat. No. 3,726,906) describe pyrolysis of tin formate compounds to form hexaalkyltin compounds. Another method of forming ditin compounds through the intermediacy of a tin formate species has also been described (Jousseaume et al.). Stemniski (U.S. Pat. No. 3,674,822) describes reacting appropriate Grignard reagents and tin dihalides to give cyclic bis(aryloxyaryl)tin compounds. Debreczeni et al. (U.S. Pat. No. 3,699,138) describe the synthesis of distannanes by the reaction of a triorganotin halide and molten sodium. Relatively short chain stannane oligomers have been reported formed by coupling various diiodo tin compounds with Ph.sub.3 SnLi (Adams et al.). Recently, a Wurtz-type coupling approach has been described for which relatively long chain polystannanes are reported (Zou et al.). Also, a synthesis of linear oligostannanes has been proposed in which a tin atom is added to a growing chain by a two-step process requiring regeneration of a terminal Sn--H group (Sita).
Commercially, organostannanes are usually formed via Wurtz or Grignard coupling reactions, even though the reagents used in these reactions are frequently pyrophoric and difficult to handle. At present, Wurtz or Grignard schemes would appear to be the methods of choice commercially in the formation of ditin species. Among the difficulties associated with synthesizing compounds having tin-tin bonds is that the bonds are easily cleaved by oxygen, halogen or acid, as well as light, as mentioned above. On the other hand, these properties of high and versatile reactivity make catenated tin compounds particularly attractive as candidates for the deposition of thin tin films.
New synthetic procedures for coupling tin species are desired, whereby compounds containing multiple Sn--Sn bonds can be formed safely and conveniently. Further desired is a method for forming catenated tin polymers, polystannanes, from relatively simple and inexpensive precursor molecules. Long-chain tin compounds formed by such a method are expected to exhibit highly advantageous photolability properties.